Any ideas on what could cause a sudden loss in mileage per gallon?

About 2 months ago, I was getting about 320 miles from full to vapors on a tank of gas... now I'm getting about 250... which is a good 20 percent less fuel economy.... this changed over the course of a month... could have been more immediate but that is when I first noticed it.

I'm not getting any codes indicating that something is wrong... and I just changed my oil/filter (running full synthetic) about a month ago (after I noticed the change)

This is on a 94 Integra LS non-vtec with no mods (save an AEM2 intake that I put on there just because I had it lying around)... but that's been on there for quite a while.

Any help would be appreciated... I'm at a loss here and this is kind of disturbing.

Edit: By the way, in my frustration, I tried using a Valvoline Fuel System cleaner but it's been a few tanks since then and it's had no effect.

 

has ur foot gotten heavier?

 

Fuel filter, spark plugs, cap & rotor etc.

 

You also might consider at least checking the oxygen sensor and coolant temperature sensor.